Rett syndrome (RTT) is caused by loss-of-function mutations in the X-linked MECP2 encoding methyl-CpG-binding protein 2 (MeCP2), an epigenetic modulator of gene transcription. RTT and the more prevalent regressive type autism share a substantial phenotypic overlap, including autistic behaviors. The affected children start to lose developed skills (called regression in this proposal) after a period of apparently normal growth and development. RTT is a disease of synaptic plasticity due to abnormal epigenetic regulation, and is potentially curable if synaptic dysfunction can be ameliorated. Critical questions for our understanding of RTT include what mechanisms trigger the regression, and if astrocytes, by regulating synaptic activities, play a role in RTT onset and progression. Our studies revealed multiple abnormalities of MeCP2-deficient astrocytes, including failure to support dendritic growth, reduced spontaneous and evoked Ca2+ signaling, aberrant responses to ATP and glutamate challenges, and abnormal D-serine release, among others. We hypothesize that the progressive astrocytic abnormalities in RTT may deteriorate the neuron-glia communications and trigger the regression. To generate data in support of this hypothesis, our aims are: 1. Investigate the mechanism of abnormal intracellular Ca2+ signaling in RTT astrocytes in vitro and in situ. We will extend our existing in vitro data and examine major cellular pathways and molecules regulating intracellular Ca2+ dynamics. We will also replicate our results in situ using hippocampal slice preparations. 2. Determine the impact of astrocytic MeCP2-deficiency on hippocampal long-term potentioation (LTP), focusing on the role of D-serine. We will measure LTP in mixed neuron-glia cultures and acute hippocampal slices to determine the contribution of MeCP2-deficient astrocytes to abnormal LTP seen in RTT samples. Subsequently, we will determine if the addition of D-serine, a gliotransmitter the release of which by MeCP2-deficient astrocytes is substantially reduced, can alleviate the LTP impairments. 3. Determine the role of astrocytic MeCP2-deficiency in the onset and progression of RTT-like symptoms in vivo. We will generate mouse models in which astrocytic Mecp2 is selectively turned off in the wild-type background or turned on in the Mecp2-null background at the time of choice. We will examine the behavioral phenotype, neuropathology and biochemical changes of these mice. The results are expected to advance our understanding of the physiological and pathological functions of astrocytes during development, provide useful models for future studies, and identify potential therapeutic targets for RTT. Lay: Rett syndrome is a genetic disorder of brain development that afflicts very young girls. Astrocytes, a type of brain cells that are active partners of neurons, are highly abnormal in an animal model of Rett syndrome. The aims of our proposal are to discover the molecular mechanisms underlying astrocytic abnormalities, and to generate new mouse models to prove the important role of astrocytes in Rett syndrome.